Asp.Net Core EF Core many to many relation update command

Question

I followed this link enter link description here to create many to many relationship. But, I do not know how to create and update Tag value to Post Object.
Any help would be appreciated.

Update, related code

class MyContext : DbContext
{
    public DbSet<Post> Posts { get; set; }
    public DbSet<Tag> Tags { get; set; }

    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        modelBuilder.Entity<PostTag>()
            .HasKey(t => new { t.PostId, t.TagId });

        modelBuilder.Entity<PostTag>()
            .HasOne(pt => pt.Post)
            .WithMany(p => p.PostTags)
            .HasForeignKey(pt => pt.PostId);

        modelBuilder.Entity<PostTag>()
            .HasOne(pt => pt.Tag)
            .WithMany(t => t.PostTags)
            .HasForeignKey(pt => pt.TagId);
    }
}

public class Post
{
    public int PostId { get; set; }
    public string Title { get; set; }
    public string Content { get; set; }

    public List<PostTag> PostTags { get; set; }
}

public class Tag
{
    public string TagId { get; set; }

    public List<PostTag> PostTags { get; set; }
}

public class PostTag
{
    public int PostId { get; set; }
    public Post Post { get; set; }

    public string TagId { get; set; }
    public Tag Tag { get; set; }
}

Update2: Code for Update record With below code, it will create records in both three tables.

    var p = new Post { Content = "C1" };
    var t = new Tag { TagId = "T1" };
    var pt = new PostTag { Post = p, Tag = t };
    _context.PostTag.Add(pt);
    _context.SaveChanges();

But, with below code, it will insert new records in middle table PostTag instead of update the previous records.

    var t1 = new Tag { TagId = "T3" };
    var t2 = new Tag { TagId = "T4" };

    var p =_context.Posts.Find(1);
    p.PostTags = new List<PostTag>() {
        new PostTag{ Post=p, Tag=t1},
        new PostTag{ Post=p, Tag=t2}
    };
    _context.Posts.Update(p);
    _context.SaveChanges();

Code works fine. What is the result are you expecting? Especially can you elaborate on "instead of update the previous records." What previous records are you updating? — Smit
– Smit, Commented Jun 11, 2017 at 7:10

Henk Holterman · Accepted Answer · 2017-06-06 05:22:55Z

1

Let's create some sample data

var p = new Post { ... };
var t = new Tag { ... };
var pt = new PostTag { Post = p, Tag = t };

This is still in-memory so we have to add it to the context, starting with:

db.Posts.Add(p);
db.Tags.Add(t);

Note that at this stage neither p nor t has any knowledge of pt. There are two ways to insert the third entity into your database.

1) The easiest way is to add the DbSet<PostTag> property and then the line

db.PostTags.Add(pt);

2) Without that extra DbSet you will need to ensure the linking from the other 2 sides. You also need to set up the List properties:

p.PostTags = new List<PostTag> { pt };
t.PostTags = new List<PostTag> { pt };

EF will now fill out any missing link fields.

edited Jun 6, 2017 at 5:22

answered Jun 5, 2017 at 6:46

Henk Holterman

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3 Comments

Edward Over a year ago

thanks, it seems I need to manually operate the middle table to insert relationship between Post and Tag, am I right? Is this simple way not available for asp.net core Configure Many-to-Many relationship using DataAnnotation which I just need to set Tag property for post? It seems not easy to update the Tags for Post.

Henk Holterman Over a year ago

I'm not sure if EF Core will do this but you'd have to start by changing the two List<PostTag> PostTags properties to List<Post> and List<Tag> respectively.

Edward Over a year ago

After check link, it is not supported currently. Sadly. Is there any better way to use update and delete on related records?

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